Poly (2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) is useful as a molding resin and possesses excellent weatherability for outdoor applications and possesses low birefringence for use as compact discs. Polycarbonates are most commonly made by the use of phosgene. However, the use of the highly toxic phosgene and organic solvents, as well as the expense of solution polymerization methods, has prompted chemists to seek out other methods of producing polycarbonates.
Melt preparation of poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) is challenging because it has a melting point much higher than its decomposition temperature. At decomposition temperatures, poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) ring-opens to produce carbon dioxide and 2-methyl-4,4-dimethyl-2-pentenal.
U.S. Pat. No. 5,171,830 discloses a melt method for making polyalkylene carbonates by reacting a glycol having at least 4 carbon atoms separating the hydroxyl groups with a diester of carbonic acid in the presence of a catalyst. This excludes 2,2,4,4-tetramethyl-1,3-cyclobutanediol, which has only 3 carbons separating the hydroxyl groups. Glycols which possess hydroxyl groups that are separated by 2 or 3 carbon atoms present a special problem for polycarbonate preparation by methods known in the art. These glycols tend to cyclize to produce five and six membered cyclic carbonates which vaporize.
Defensive Publication T873,016 discloses a method of producing low molecular weight poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) by self-condensing the diethyl ester of 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Defensive Publication T873,016 also discloses a method of reacting the glycol with diphenyl carbonate, which results in the formation of a colored polymer.
Methods for producing polycarbonates disclosed in the art produce low molecular weight polycarbonates when 2,2,4,4-tetramethyl-1,3-cyclobutanediol is employed as the glycol reactant. Japanese Patent 62-155370 discloses a process of reacting dialkyl carbonates with a glycol in the presence of a titanium catalyst. U.S. Pat. No. 3,022,272 discloses a process of reacting a dialkyl carbonate with a glycol. Diphenyl carbonate is disclosed as the preferred dialkyl carbonate, even though aromatic carbonates produce colored polymers. Lastly, U.S. Pat. No. 3,313,777 discloses a method of producing low molecular weight poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) by reacting dibutyl carbonate with the glycol. Moreover, the resultant polymer solidified in the reaction vessel, requiring very impractical recovery measures to be taken.
In previous polycarbonate processes, dimethyl carbonate has not been used in the preparation of polycarbonates, even though it is the least expensive and most readily available dialkyl carbonate known in the art. Dimethyl carbonate is the only dialkyl carbonate to form an azeotrope with its alkanol.
U.S. Pat. No. 3,335,111 discloses a melt method of continuously preparing aromatic high molecular weight polycarbonates by reacting a dihydroxy aromatic organic compound with a monomeric biscarbonate ester of a dihydroxy compound at a temperature of from about 200.degree. C. to 325.degree. C. in a succession of reaction zones. Also disclosed is a less preferred method of condensing biscarbonate esters of dihydroxy compounds to form polycarbonate. The polymerization catalysts disclosed for the process of U.S. Pat. No. 3,335,111 are basic metal salts such as hydrides of metals of Groups I, II and III; oxides, hydroxides, phenoxides, and alkoxides of metals of Groups I and II; and metal alkyls of Groups I, II, III and IVA metals.
Defensive Publication T858,012 discloses a melt method of preparing the polycarbonate of 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) by reacting TMCD with 2,2,4,4-tetramethyl-1,3-cyclobutane bis(ethyl carbonate) in the presence of a dibutyltin oxide catalyst. However, this process produces low molecular weight discolored polymers having inherent viscosities of less than about 0.4 dL/g.
The melt production of poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) from 2,2,4,4-tetramethyl-1,3-cyclobutanediol is difficult due to the thermal decomposition of TMCD in the presence of basic catalysts. In light of the above, it would be desirable to have a melt process for economically producing high molecular weight poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate).